This invention relates to optical recording media.
For optical recording media of magneto-optical memory type, there are well known a number of materials for a recording layer thereof, for example, MnBi, MnAlGe, MnSb, MnCuBi, GdFe, TbFe, GdCo, PtCo, TbCo, TbFeCo, GdFeCo, TbFeO.sub.3, GdIG (gadolinium iron garnet), GdTbFe, GdTbFeCoBi, CoFe.sub.2 O.sub.4, etc. These materials are deposited on transparent substrates of plastic material or glass as a thin film by any suitable thin-film forming techniques such as vacuum deposition or sputtering. The features common to these magneto-optical recording thin film layers are that the axis of easy magnetization is perpendicular to the film surface and that Kerr and Farady effects are great.
Requirements imposed on such magneto-optical recording media are:
(1) that the Curie point is of the order of 100.degree. to 200.degree. C. and the compensation point is close to room temperature,
(2) that noise-inducing defects such as grain boundaries are relatively fewer, and
(3) that a magnetically and mechanically uniform film is obtained over a relatively large area.
In the light of these requirements, a great attention is recently drawn to amorphous perpendicular magnetizable thin films of rare earth element-transition metal among the above-mentioned materials. Magneto-optical recording media having such amorphous perpendicular magnetizable thin films of rare earth element-transition metal, however, have a storage problem. If the magnetic thin film layers are stored in contact with the ambient atmosphere, rare earth elements therein are preferentially erroded or oxidized by oxygen and moisture in the atmosphere, losing the necessary information recording and reproducing ability.
For this reason, most investigations are generally directed to those recording media of the construction having a protective layer disposed on a surface of a magnetic thin film layer adjacent to or remote from the substrate. Known protective layers are vacuum deposited films of inorganic materials such as silicon monoxide, silicon dioxide, aluminum nitride, silicon nitride and zinc sulfide as well as resinous coatings (see Japanese Patent Application Kokai No. 58-80142). However, these protective layers are insufficient in moisture protection or adhesion to the substrate.
A protective layer of an inorganic glass based composition is also known as achieving some improvements in durability of the medium, particularly prevention of moisture penetration and adhesion improvement. Japanese Patent Application Kokai No. 59-162621 discloses a glass layer vacuum deposited on a plastic substrate, the glass comprising 80 to 90% by weight of SiO.sub.2, 5 to 15% by weight of B.sub.2 O.sub.3, 1 to 10% by weight of Al.sub.2 O.sub.3, and 1 to 10% by weight of Na.sub.2 O. Japanese Patent Application Kokai No. 60-177449 discloses a thin film layer of inorganic glass interposed between a substrate and a recording layer. The glass is typically an alkali-free glass composition commercially available from Corning Glass as Code No. 7059 having the composition of 49.9% SiO.sub.2, 10.5% B.sub.2 O.sub.3, 4.3% CaO, 25.1% BaO, and 10.3% Al.sub.2 O.sub.3.
These approaches, however, do not fully meet the strict requirements of durability and corrosion resistance on magneto-optical recording media. There is a need for a magneto-optical recording medium having more improved properties.
The same problem arises in a magneto-optical recording medium having a recording layer of the so-called phase conversion type.